A hyperspectral imaging method was
developed that allowed the identification
of heterogeneous plasmon response from 50 nm diameter gold colloidal
particles on a conducting substrate in a transparent three-electrode
spectroelectrochemical cell under non-Faradaic conditions. At cathodic
potentials, we identified three distinct behaviors from different
nanoparticles within the same sample: irreversible chemical reactions,
reversible chemical reactions, and reversible charge density tuning.
The irreversible reactions in particular would be difficult to discern
in alternate methodologies. Additional heterogeneity was observed
when single nanoparticles demonstrating reversible charge density
tuning in the cathodic regime were measured dynamically in anodic
potential ranges. Some nanoparticles that showed charge density tuning
in the cathodic range also showed signs of an additional chemical
tuning mechanism in the anodic range. The expected changes in nanoparticle
free-electron density were modeled using a charge density-modified
Drude dielectric function and Mie theory, a commonly used model in
colloidal spectroelectrochemistry. Inconsistencies between experimental
results and predictions of this common physical model were identified
and highlighted. The broad range of responses on even a simple sample
highlights the rich experimental and theoretical playgrounds that
hyperspectral single-particle electrochemistry opens.
Strong light-absorbing
properties allow plasmonic metal nanoparticles
to serve as antennas for other catalysts to function as photocatalysts.
To achieve plasmonic photocatalysis, the hot charge carriers created
when light is absorbed must be harnessed before they decay through
internal relaxation pathways. We demonstrate the role of photogenerated
hot holes in the oxidative dissolution of individual gold nanorods
with millisecond time resolution while tuning charge-carrier density
and photon energy using snapshot hyperspectral imaging. We show that
light-induced hot charge carriers enhance the rate of gold oxidation
and subsequent electrodissolution. Importantly, we distinguish how
hot holes generated from interband transitions versus hot holes around
the Fermi level contribute to photooxidative dissolution. The results
provide new insights into hot-hole-driven processes with relevance
to photocatalysis while emphasizing the need for statistical descriptions
of nonequilibrium processes on innately heterogeneous nanoparticle
supports.
Plasmonic nanoparticles offer promise in photoelectrochemistry by enhancing the rate and selectivity of reactions and in sensing by responding optically to local reactions. Optical electrochemical measurements at the single-particle level are necessary for understanding and eventually controlling the role of plasmons in such complex environments. Recently, researchers have developed techniques to optically measure electrochemical reactions at the surface of single nanoparticles and individual aggregates, allowing for the high-throughput screening necessary to resolve subpopulations of active nanoparticle catalysts and identify active sites on aggregate structures. This review highlights single-nanoparticle and nanoparticle aggregate electrochemical techniques and how they can be used to isolate and elucidate the role of surface plasmons in enhancing catalyst activity and sensing electrochemical processes at the nanoscale.
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